1. Field of the Invention
The invention relates to a Coriolis mass flow meter. More particularly, it relates to a Coriolis mass flow meter comprising a pair of counter-oscillating U-shaped measuring tubes.
2. Prior Art
FIGS. 1A, 1B, 2A, 2B, and 2C show a Coriolis mass flow meter according to the prior art. FIG. 1A is a perspective view, and FIG. 1B is a diagrammatic side view. The Coriolis mass flow meter 1 comprises a housing or a metallic frame 12, at the ends of which an inlet 3 and an outlet 4 for a fluid medium are located. In the vicinity of the inlet 3 and the outlet 4 there are located process connectors 13 for the purpose of incorporating the meter 1 in a process line. Between the inlet 3 and the outlet 4 there is situated a pair of U-shaped measuring tubes 5, each of which is capable of transporting a fluid medium. To ensure that both measuring tubes 5 will transport fluid, a stream-splitting device 11 is provided in the present case in the vicinity of each of the inlet 3 and the outlet 4. Both of the measuring tubes 5 have a central arch 6 and a region nearer to the end of the U-shaped portion, by means of which the two U-shaped measuring tubes are connected to the inlet 3 or the outlet 4. In the latter region there are provided in each case two joint plates 7, 7′ and 8, 8′, which determine the positions of the two U-shaped measuring tubes in relation to each other. The flow axis for the medium is designated by d.
For the purpose of measuring a mass flow by means of the Coriolis mass flow meter 1, the two measuring tubes 5 are caused to oscillate in opposite directions by means of a vibration exciter 9. The two U-shaped measuring tubes then move periodically away from each other and back towards each other. The oscillatory movements of the measuring tubes 5 are then detected by the two vibration sensors 10 and 10′. If no fluid flows through the Coriolis mass flow meter 1, the movements of the measuring tubes 5 detected by the two vibration sensors 10 and 10′ respectively are in phase with each other. When, on the other hand, a fluid medium is flowing through the measuring tubes 5, the medium flowing therethrough will then experience, on account of the oscillatory movement of the measuring tubes 5, a Coriolis force that is differently vectored in the region of the respective vibration sensors 10 and 10′ respectively. For this reason, a phase shift occurs between the signals detected by means of the vibration sensor 10 and 10′ respectively. The measured phase angle is directly proportional to the mass flow. The modulus of elasticity of the measuring tube material is temperature-dependent and is likewise included in the proportionality constant between phase angle and mass flow. For this reason, the temperature of the measuring tubes 5 is measured and the phase angle adapted accordingly. With the aid of suitable signal processing techniques, a usable signal is formed from the measured signals, and this provides information on the desired mass flow.
The prior Coriolis mass flow meters are also used for media of low density (e.g. gases) or for media of very high viscosity, that is to say, for media involving a very low mass flow. Besides, the detected phase angle is generally rather small. Thus to ensure that even small mass flows can be measured precisely, a high zero point stability of the Coriolis mass flow meter is required. This is generally achieved by arranging for the Coriolis mass flow meters to be as free of vibration as possible in the region of the process connectors so that no vibrational energy can propagate into the adjoining process line. If the forces generated by the vibrating measuring tubes in the joint plates are not completely compensated for, the meter will then vibrate as a whole so as to excite the adjoining process line, so that feedbacks will occur and the zero point will become unstable. This is particularly a problem when there is a resonance frequency in the process line itself or in superstructures installed therein, such as boilers for example, the resonance frequency is similar to that of the Coriolis mass flow meter. In this case there will occur significant interactions between the device and the environment and the zero point will quickly become unstable. This leads to measuring errors.
In order to increase the zero point stability on prior Coriolis mass flow meters, it is known to use joint plates. These connect the U-shaped measuring tubes in the vicinity of the ends of the leg of the U to each other such that the position of the measuring tubes in relation to each other remains fixed. They serve the task of separating the natural, self-exited vibration of the measuring tubes, as occurs with non-flowing fluid, from the vibration based on Coriolis forces as occurs with flowing fluid and to the task of reducing the transfer of vibration between the measuring tubes and the piping system. Attempted solutions are disclosed in EP 1 166 051 B1, EP 1 985 975 A2, and WO 2009/050133 A1. In each case, two joint plates are provided at each leg end of the U-shaped measuring tubes, and the position of said joint plates is precisely defined. More particularly, in EP 1 985 975 A2 the attempt is made to adapt the arrangement of the joint plates by means of FEM (Finite Element Method) and thus to minimize the vibration amplitude at the process connectors of the device.
EP 1 248 084 A1 discloses a Coriolis mass flow detector comprising two curved measuring tubes that are disposed symmetrically about a plane of symmetry E and oppose each other in a mirror-inverted manner and are caused to oscillate substantially at right angles to said plane of symmetry. The two measuring tubes are disposed such that their sectional planes enclose an angle α of less than 3°, in order to compensate for forces present in the plane of symmetry E and occurring at the tube ends.
However, it has been found that there is still room for improvement as regards the zero point stability in spite of the solutions already proposed.